At present, a lot of literatures regarding filters used for filtering molten metal are available.
U.S. Pat. No. 5,104,540 (Corning Inc.) discloses a carbon-coated porous sintered ceramic filter for filtering molten metal, comprising a single unit filter substrate material formed from refractory material such as alumina, mullite, zircon powder, zirconia, spinel, cordierite, lithium, aluminosilicate, titanate, feldspar, quartz, fumed silica, silicon carbide, kaolin, aluminum titanate, silicate, aluminate and mixtures thereof. A carbon-based coating is applied to the surface of the filter screen or used as a thermite.
U.S. Pat. No. 5,520,823 discloses a filter only for filtering molten aluminum, wherein borosilicate glass is employed as the binder. Although the filter contains graphite, considerable amount of graphite is lost due to sintering in the air. The loss of carbon limits this filter to be used in filtering aluminum metal only while is not suitable for filtration of molten iron or steel.
WO 0218075 discloses a filter for filtering molten metal, wherein the filter comprises open cell porous material containing refractory particles bonded together by a binder containing carbon structure. That is to say, in this filter, no other bonding mechanism except for carbon binder exists.
In the present filters, the high temperature resistance of the filter is enhanced by the employment of carbon binder, while the content thereof needs to be controlled within a narrow range, which is not favourable for controlling the contents of the binder during the processing. Furthermore, although carbon containing refractory materials can withstand temperature higher than that of the molten metal, prevent metal penetration, exhibit very high strength at high temperature and show better thermal shock resistance; unfortunately, the filter bonded by carbon binder has very low strength at room temperature, at the same time, it tends to absorb moisture, thus influencing the use of this product under high temperature.